The present invention relates generally to radio receivers adapted to receive and process wideband impulse radio signals. More particularly, this invention pertains to devices and circuits for accurately converting in an impulse radio receiver a series of time-modulated radio pulses into a baseband signal.
There is a continuing need for the development of advanced wireless devices for communications of voice and data, for materials measurement, navigation, environmental sensing, radar, security and numerous other civilian and military applications of radio technology. Improvements are needed in the underlying technology to provide greater reliability, greater accuracy, lower power consumption, lower cost, reduced size, and efficient use of the limited available spectrum. Conventional narrow band AM, FM, CDMA, TDMA and similar wireless communications methods and systems have not fully met these needs.
However, there is an emerging technology called Impulse Radio (including Impulse Radar) (“IR”) that offers many potential advantages in addressing these needs. Impulse radio was first fully described in a series of patents including U.S. Pat. No. 4,641,317 (issued Feb. 3, 1987), U.S. Pat. No. 4,813,057 (issued Mar. 14, 1989), U.S. Pat. No. 4,979,186 (issued Dec. 18, 1990) and U.S. Pat. No. 5,303,108 (issued Nov. 8, 1994), all invented by Larry W. Fullerton and assigned to Time Domain Corporation. The disclosure of each of these patents is incorporated in this patent specification by reference.
Impulse radio systems are generally characterized by their transmission of short duration broad band pulses on a relatively low duty cycle. In some systems these pulses may approach a Gaussian monocycle, where the instantaneous pulse bandwidth is on the order of the center frequency. The short pulse, low duty cycle mechanism produces a processing gain that may be utilized for interference rejection and channelization. Because of the extremely wide instantaneous bandwidth of the pulse, the available processing gain far exceeds what is achieved using typical conventional spread spectrum methods. This enables the utilization of many more channels at higher dynamic ranges and higher data rates than are available in the typical conventional spread spectrum system.
Impulse radio systems have further advantages in the resistance to multipath effect. Because impulse radio signals are divided in time rather than in frequency, time related effects, such as multipath interference, can be separated, resulting in lower average power and higher reliability for a given power level.
Impulse radio techniques are also useful in radar systems. Impulse radar systems enjoy the combined advantages of very short pulses at relatively low frequencies. The short pulses result in high resolution and the low frequency gives relatively high material penetration. If a radar system used a pulse of equivalent bandwidth at a higher carrier frequency, the material penetration properties would usually be impaired. This combined advantage enables IR to be used for ground penetrating radar for inspection of bridges, roads, runways, utilities and the like, and security applications, and to “see” through walls radar for emergency management situations.
Existing IR receivers typically use mixer or sampling technology which is large in size, inefficient in power consumption and which is difficult to reproduce in a manufacturing environment. This results in a high cost to the user. Improvements are thus needed in converter technology to reduce size, weight, power consumption and cost and to improve the manufacturing yield and reliability of these systems.
Impulse radio systems are not limited to transmitting and receiving Gaussian monocycle pulses. However, some basic impulse radio transmitters attempt to emit short Gaussian monocycle pulses having a tightly controlled average pulse-to-pulse interval. A Gaussian monocycle is the first derivative of the Gaussian function. However, in a real world environment, a perfect Gaussian pulse is not achievable. In the frequency domain, this results in a slight reduction in the signal bandwidth. The signals transmitted by an IR transmitter, including Gaussian monocycles, signals having multiple cycles in a Gaussian envelope, and their real world variations, are sometimes called impulses.
The Gaussian monocycle waveform is naturally a wide bandwidth signal, with the center frequency and the bandwidth dependent on the width of the pulse. The bandwidth is approximately 160% of the center frequency. In practice, the center frequency of a monocycle pulse is approximately the reciprocal of its length, and its bandwidth is approximately equal to 1.6 times the center frequency. However, impulse radio systems can be implemented where the transmitted and/or received signals have waveforms other than an ideal Gaussian monocycle.
Most prior art wireless communications systems use some variation of amplitude modulation (AM) or frequency modulation (FM) to communicate voice or data with a radio carrier signal. However, impulse radio systems can communicate information using a novel technique known as pulse position modulation. Pulse position modulation is a form of time modulation in which the value of each instantaneous value or sample of a modulating signal (e.g., a voice or data signal) is caused to change or modulate the position in time of a pulse. In the frequency domain, pulse position modulation distributes the energy over more frequencies.
In some impulse radio communications, the time position (pulse-to-pulse interval) is preferably varied on a pulse-by-pulse basis by two separate components: an information component and a pseudo-random code component. Prior art spread spectrum radio systems make use of pseudo-random codes to spread a narrow band information signal over a relatively wide band of frequencies. A spread spectrum receiver then correlates these signals to retrieve the original information signal. Unlike conventional spread spectrum systems, impulse radio systems do not need the pseudo-random code for energy spreading. In some applications, impulse radio transmitters can use pulse widths of between 20 and 0.1 nanoseconds (ns) and pulse-to-pulse intervals of between 2 and 5000 ns. These narrow monocycle pulses have an inherently wide information bandwidth. (The information bandwidth, also referred to simply as the “bandwidth”, is the range of frequencies in which one or more characteristics of communications performance fall within specified limits.)
Thus, in some impulse radio systems, the pseudo-random (PN) code component is used for different purposes: channelization; energy smoothing in the frequency domain; and interference resistance. Channelization is a procedure employed to divide a communications path into a number of channels. In a system that does not use a coding component, differentiating between separate transmitters would be difficult. PN codes create channels, if there is low correlation and/or interference among the codes being used. If there were a large number of impulse radio users within a confined area, there might be mutual interference. Further, while the use of the PN coding minimizes that interference, as the number of users rises the probability of an individual pulse from one user's sequence being received simultaneously with a pulse from another user's sequence increases. Fortunately, impulse radio systems can be designed so that they do not depend on receiving every pulse. In such systems, the impulse radio receiver can perform a correlating, synchronous receiving function (at the RF level) that uses a statistical sampling of many pulses to recover the transmitted information. Advanced impulse radio systems may utilize multiple pulses to transmit each data bit of information, and each pulse may be dithered in time to further smooth the spectrum to reduce interference and improve channelization. These systems may also include a sub-carrier for improved interference resistance and implementation advantages. In other embodiments of an impulse radio system, however, each “bit” of transmitted information can be represented by a single pulse, with no coding component.
Energy smoothing in the frequency domain insures that impulse radio transmissions interfere minimally with conventional radio systems. In some impulse radio systems, optimal energy smoothing is obtained by applying to each pulse a PN code component dither having a much larger magnitude than the information component dither.
Besides channelization and energy smoothing, the PN coding can also makes impulse radio highly resistant to interference from all radio communications systems, including from other impulse radio transmitters. This is critical, as any other signals within the band occupied by an impulse signal can act as interference to the impulse radio. Because there are no unallocated bands at or above 1 GHz available for impulse radio systems, they must share spectrum with other conventional and impulse radios without being adversely affected. Using a PN code can help impulse systems discriminate between the intended impulse transmission and transmissions from others.
In many IR systems, the impulse radio receiver is a direct conversion receiver with a single conversion stage that coherently converts a series of pulses into a baseband signal. The baseband signal is the information channel for the basic impulse radio communications system. In such systems, pulse trains, not single pulses, are used for communications. Accordingly, the impulse radio transmitter in such systems generates a train of pulses for each bit of information. The data rate of such an impulse radio transmission is only a fraction of the periodic timing signal used as a time base. Each data bit modulates the time position of many of the pulses of the periodic timing signal. This yields a modulated, coded timing signal that comprises a train of identical pulses for each single data bit. Some impulse radio receivers typically integrate 200 or more pulses to yield the baseband output. Other systems use a “one pulse per bit” information transmission scheme. The number of pulses over which the receiver integrates is dependent on a number of variables, including pulse rate, bit rate, interference levels, and range.
A block diagram of one embodiment of a basic impulse radio receiver 100 is shown in FIG. 7. The receiver 100 includes a receive antenna 56 for receiving a propagated impulse radio signal 101. The received signal is sent to a baseband signal converter 10 via a receiver transmission line 102, coupled to the receive antenna 56.
The receiver 100 also includes a decode timing modulator/decode source 55 and an adjustable time base 57. The adjustable time base 57 can be a voltage-controlled oscillator or, as shown, a variable delay generator 52 coupled to the output of a time base 51. The decode timing modulator/decode source 55 generates a primary timing pulse (decode signal 103) corresponding to the PN code used by the associated impulse radio transmitter (not shown) that transmitted the propagated signal 101. The adjustable time base 57 generates a periodic timing signal having a train of template signal pulses with waveforms substantially equivalent to each pulse of the received signal 101.
The baseband signal conversion process performed by the converter 10 includes a cross-correlation operation of the received signal 101 with the decode signal 103. Integration over time of the cross-correlated received signal generates a baseband signal 104. The baseband signal 104 is then demodulated by a demodulator 50 to yield a demodulated information signal 105. The demodulated information signal 105 is substantially identical to the information signal of the transmitter that sent the received signal 101.
The baseband signal 104 is also coupled to a low pass filter 53. The low pass filter 53 generates an error signal 106 for an acquisition and lock controller 54 to provide minor timing adjustments to the adjustable time base 57.
As noted above, the circuit or device in an impulse radio receiver that converts the received impulses into a baseband signal is sometimes referred to as a cross-correlator or sampler. The baseband signal converter of an impulse radio receiver integrates one or more pulses to recover the baseband signal that contains the transmitted information. One embodiment of a cross-correlator device usable in an impulse radio receiver is described in U.S. Pat. No. 5,677,927, issued Oct. 14, 1997, and assigned to Time Domain Corporation. The disclosure of the '927 Patent is incorporated in this specification by reference.
Unfortunately, prior art baseband signal converter devices and circuits have not been entirely satisfactory or are subject to inherent performance limitations. In general, such converter devices have been constructed from discrete electronic components. The deficiencies inherent in discrete circuit designs include high power consumption, excessive device size, and a need for careful matching and/or “fine tuning” of component values and/or operational parameters to produce accurate and consistent performance. For example, the converter circuit described in FIG. 2a of U.S. Pat. No. 4,979,186 uses a sampling bridge requiring four diodes that must be carefully matched in performance characteristics. Similarly, the converter circuit design shown in FIG. 3 of the '186 patent can produce a performance-degrading signal offset that varies over time and temperature. Moreover, the use of discrete electronic components in the converter device places undesirable limits on the switching speeds of the active components used in the circuits, making it more difficult to perform the signal conversion process using very short sample times.
A further issue that has not been satisfactorily addressed by prior art baseband signal converter designs is flexibility in application. Some important impulse radio applications can be enabled or enhanced by concurrently operating multiple baseband converter circuits in a single receiver. Scanning and rake receivers are examples of impulse radio applications where the use of two or more baseband signal converters in a single receiver would be highly desirable. Unfortunately, a baseband signal converter device that integrates multiple converter circuits in a single, low profile package has not been available in the prior art.
What is needed, then, is low profile, low power integrated circuit device containing one or more circuits that can convert time-modulated radio pulses into a baseband signal, and that is capable of executing the conversion process accurately and consistently over time and temperature using a short sample period.